Since an organic EL device is a self light-emitting device, it is brighter, better in visibility, and capable of clearer display, than a liquid crystal device. Hence, energetic researches have been conducted on organic EL devices.
In 1987, C. W. Tang et al. of Eastman Kodak Company developed a laminated structure device sharing various roles among different materials, thereby imparting practical applicability to organic EL devices using organic materials. Such an organic EL device is formed by laminating a layer of a fluorescent body capable of transporting electrons, and a layer of an organic substance capable of transporting holes. Because of this configuration, the organic EL device is adapted to inject positive charges and negative charges into the layer of the fluorescent body to perform light emission, thereby obtaining a high luminance of 1,000 cd/m2 or more at a voltage of 10V or less (see Patent Document 1 and Patent Document 2)
Many improvements have been made to date for commercialization of organic EL devices. For example, high efficiency and high durability have been achieved by an electroluminescent device having a laminated structure, in which the roles of the respective layers are shared among more types of materials, and having an anode, a hole injection layer, a hole transport layer, a luminous layer, an electron transport layer, an electron injection layer, and a cathode provided on a substrate.
For a further increase in the luminous efficiency, it has been attempted to utilize triplet excitons, and the utilization of phosphorescent luminous compounds has been considered. Furthermore, devices utilizing light emission by thermally activated delayed fluorescence (TADF) have been developed. Adachi et al. from Kyushu University realized in 2011 an external quantum efficiency of 5.3% by a device using a thermally activated delayed fluorescence material.
The luminous layer can also be prepared by doping a charge transporting compound, generally called a host material, with a fluorescent compound, a phosphorescent luminous compound, or a material radiating delayed fluorescence. The selection of the organic material in the organic EL device greatly affects the characteristics of the device, such as efficiency and durability.
With the organic EL device, the charges injected from both electrodes recombine in the luminous layer to obtain light emission. For this purpose, how efficiently the charges of the holes and the electrons are passed on to the luminous layer is of importance, and the device needs to be excellent in carrier balance. Moreover, the hole injecting properties are enhanced, and the electron blocking properties of blocking electrons injected from the cathode are enhanced, whereby the probability of the holes and the electrons recombining is increased. Besides, excitons generated within the luminous layer are confined. By so doing, a high luminous efficiency can be obtained. Thus, the role of the hole transport material is so important that there has been a desire for a hole transport material having high hole injection properties, allowing marked hole mobility, possessing high electron blocking properties, and having high durability to electrons.
From the viewpoint of device life, heat resistance and amorphousness of the material are also important. A material with low heat resistance is thermally decomposed even at a low temperature by heat produced during device driving, and the material deteriorates. With a material having low amorphousness, crystallization of a thin film occurs even in a short time, and the device deteriorates. Thus, high resistance to heat and satisfactory amorphousness are required of the material to be used.
As hole transport materials so far used for organic EL devices, N,N′-diphenyl-N,N′-di(α-naphthyl)benzidine (NPD) and various aromatic amine derivatives have been known (see Patent Document 1 and Patent Document 2). NPD has satisfactory hole transport capability, but its glass transition temperature (Tg) serving as an index of heat resistance is as low as 96° C. Under high temperature conditions, therefore, device characteristics decline because of crystallization.
Among the aromatic amine derivatives described in Patent Documents 1 and 2 are compounds having excellent hole mobility of 10−3 cm2/Vs or more. Since the electron blocking properties of such aromatic amine derivatives are insufficient, however, some of electrons pass through the luminous layer, and an increase in the luminous efficiency cannot be expected. Thus, there has been a desire for a material having higher electron blocking properties, more stable in the form of a thin film, and possessing higher resistance to heat, in order to achieve an even higher efficiency.
In addition, highly durable aromatic amine derivatives have been reported in Patent Document 3. However, the aromatic amine derivatives of Patent Document 3 have been used as charge transport materials in electrophotographic photoreceptors, and examples of using them as organic EL devices have not been studied at all.
As compounds improved in characteristics such as heat resistance and hole injection properties, arylamine compounds having substituted carbazole structures have been proposed (see Patent Document 4 and Patent Document 5). In devices using these compounds as hole injection layers or hole transport layers, heat resistance and luminous efficiency have been improved. However, the improved characteristics have been still insufficient, and an even lower driving voltage and an even higher luminous efficiency are desired.
In the field of organic EL devices, as discussed above, it has been attempted to increase the yield of device preparation and improve the device characteristics, by combining materials excellent in the hole injection/transport performance, the electron injection/transport performance, thin film stability, and durability. Through these efforts, it is desired to realize a device, which is satisfactory in carrier balance, enables holes and electrons to recombine with high efficiency, has a high luminous efficiency, works at a low driving voltage, and has a long life.